5.1 An Inventory of the Solar System

The solar system consists of the Sun and many smaller objects: the planets, their moons and rings, and such “debris” as asteroids, comets, and dust. Decades of observation and spacecraft exploration have revealed that most of these objects formed together with the Sun about 4.5 billion years ago. They represent clumps of material that condensed from an enormous cloud of gas and dust. The central part of this cloud became the Sun, and a small fraction of the material in the outer parts eventually formed the other objects.

During the past 50 years, we have learned more about the solar system than anyone imagined before the space age. In addition to gathering information with powerful new telescopes, we have sent spacecraft directly to many members of the planetary system. (Planetary astronomy is the only branch of our science in which we can, at least vicariously, travel to the objects we want to study.) With evocative names such as Voyager, Pioneer, Curiosity, and Pathfinder, our robot explorers have flown past, orbited, or landed on every planet, returning images and data that have dazzled both astronomers and the public. In the process, we have also investigated two dwarf planets, hundreds of fascinating moons, four ring systems, a dozen asteroids, and several comets (smaller members of our solar system that we will discuss later).

Our probes have penetrated the atmosphere of Jupiter and landed on the surfaces of Venus, Mars, our Moon, Saturn’s moon Titan, the asteroids Eros and Itokawa, and the Comet Churyumov-Gerasimenko (usually referred to as 67P). Humans have set foot on the Moon and returned samples of its surface soil for laboratory analysis as shown in Figure 5.1. We have even discovered other places in our solar system that might be able to support some kind of life.

Astronauts on the Moon

Photograph of Astronauts on the Moon. At centre is the landing module, and to the right is the Lunar rover used by the Astronauts to travel large distances from the landing site. At left an Astronaut salutes the American flag placed near the lander. Scattered throughout the foreground are footprints in the Lunar soil.
Figure 5.1. The lunar lander and surface rover from the Apollo 15 mission are seen in this view of the one place beyond Earth that has been explored directly by humans.
Apollo 15 flag, rover, LM, Irwin by NASA/David R. Scott, modified by Bammesk, NASA Media Licence.

An Inventory

The Sun, a star that is brighter than about 80% of the stars in the Galaxy, is by far the most massive member of the solar system, as shown in Table 5.1. It is an enormous ball about 1.4 million kilometres in diameter, with surface layers of incandescent gas and an interior temperature of millions of degrees. The Sun will be discussed in later chapters as our first, and best-studied, example of a star.

Table 5.1. Mass of Members of the Solar System
Object Percentage of Total Mass of Solar System
Sun 99.80
Jupiter 0.10
Comets 0.0005–0.03 (estimate)
All other planets and dwarf planets 0.04
Moons and rings 0.00005
Asteroids 0.000002 (estimate)
Cosmic dust 0.0000001 (estimate)

Table 5.1 also shows that most of the material of the planets is actually concentrated in the largest one, Jupiter, which is more massive than all the rest of the planets combined. Astronomers were able to determine the masses of the planets centuries ago using Kepler’s laws of planetary motion and Newton’s law of gravity to measure the planets’ gravitational effects on one another or on moons that orbit them. Today, we make even more precise measurements of their masses by tracking their gravitational effects on the motion of spacecraft that pass near them.

Beside Earth, five other planets were known to the ancients—Mercury, Venus, Mars, Jupiter, and Saturn—and two were discovered after the invention of the telescope: Uranus and Neptune. The eight planets all revolve in the same direction around the Sun. They orbit in approximately the same plane, like cars travelling on concentric tracks on a giant, flat racecourse. Each planet stays in its own “traffic lane,” following a nearly circular orbit about the Sun and obeying the “traffic” laws discovered by Galileo, Kepler, and Newton. Besides these planets, we have also been discovering smaller worlds beyond Neptune that are called trans-Neptunian objects or TNOs as shown in Figure 5.2. The first to be found, in 1930, was Pluto, but others have been discovered during the twenty-first century. One of them, Eris, is about the same size as Pluto and has at least one moon (Pluto has five known moons.) The largest TNOs are also classed as dwarf planets, as is the largest asteroid, Ceres. To date, more than 1750 of these TNOs have been discovered.

Orbits of the Planets

Diagram of the Orbits of the Planets and the five known Dwarf-Planets. The orbits of each object is shown as a blue ellipse. All eight major planets and the asteroids orbit the Sun in roughly the same plane, but the orbits of the outer dwarf planets do not. The objects plotted in the diagram moving outward from the Sun are Mercury, Venus, Earth, Mars, Ceres, Jupiter, Saturn, Uranus, Neptune, Pluto, Haumea, Makemake, and Eris.
Figure 5.2. All eight major planets orbit the Sun in roughly the same plane. The five currently known dwarf planets are also shown: Eris, Haumea, Pluto, Ceres, and Makemake. Note that Pluto’s orbit is not in the plane of the planets. Image by Open Stax CC BY 4.0

Each of the planets and dwarf planets also rotates (spins) about an axis running through it, and in most cases the direction of rotation is the same as the direction of revolution about the Sun. The exceptions are Venus, which rotates backward very slowly (that is, in a retrograde direction), and Uranus and Pluto, which also have strange rotations, each spinning about an axis tipped nearly on its side. We do not yet know the spin orientations of Eris, Haumea, and Makemake.

The four planets closest to the Sun (Mercury through Mars) are called the inner or terrestrial planets.  Often, the Moon is also discussed as a part of this group, bringing the total of terrestrial objects to five. (We generally call Earth’s satellite “the Moon,” with a capital M, and the other satellites “moons,” with lowercase m’s.) The terrestrial planets are relatively small worlds, composed primarily of rock and metal. All of them have solid surfaces that bear the records of their geological history in the forms of craters, mountains, and volcanoes as shown in Figure 5.3.

Surface of Mercury

Image of the surface of Mercury taken from Mariner 10. Large craters, with many overlapping one upon the other, cover the surface of this 400 km wide scene.
Figure 5.3. The pockmarked face of the terrestrial world of Mercury is more typical of the inner planets than the watery surface of Earth. This black-and-white image, taken with the Mariner 10 spacecraft, shows a region more than 400 kilometres wide.
PIA16757 by NASA/John Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington, JPL Media Licence.

The next four planets (Jupiter through Neptune) are much larger and are composed primarily of lighter ices, liquids, and gases. We call these four the jovian planets (after “Jove,” another name for Jupiter in mythology) or giant planets—a name they richly deserve. They are shown in Figure 5.4. More than 1400 Earths could fit inside Jupiter, for example. These planets do not have solid surfaces on which future explorers might land. They are more like vast, spherical oceans with much smaller, dense cores.

The Four Giant Planets

Diagram of the Four Giant Planets Shown to Scale. Arranged from left to right are Jupiter, Saturn, Uranus, and Neptune. Also shown to scale at lower centre is the Earth.
Figure 5.4. This montage shows the four giant planets: Jupiter, Saturn, Uranus, and Neptune. Below them, Earth is shown to scale.
First and Farthest by NASA, Solar System Exploration, NASA Media Licence.

Near the outer edge of the system lies Pluto, which was the first of the distant icy worlds to be discovered beyond Neptune (Pluto was visited by a spacecraft, the NASA New Horizons mission, in 2015. Figure 5.5 is the image it captured during its visit). Table 5.2 summarizes some of the main facts about the planets.

Pluto Close-up

Image of a portion of the surface of Pluto. In this photograph from New Horizons, the smooth, white Sputnik plains are seen covering most of the upper right of the image. Rugged, heavily cratered terrain covers the lower centre and upper left.
Figure 5.5. This intriguing image from the New Horizons spacecraft, taken when it flew by the dwarf planet in July 2015, shows some of its complex surface features. The rounded white area is temporarily being called the Sputnik Plain, after humanity’s first spacecraft.
Sputnik Planum, in Color by NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute, JPL Media Licence.
Table 5.2. The Planets
Name Distance from Sun Revolution Period Diameter Mass Density
(AU) (y) (km) (1023 kg) (g/cm3)
Mercury 0.39 0.24 4,878 3.3 5.4
Venus 0.72 0.62 12,120 48.7 5.2
Earth 1.00 1.00 12,756 59.8 5.5
Mars 1.52 1.88 6,787 6.4 3.9
Jupiter 5.20 11.86 142,984 18,991 1.3
Saturn 9.54 29.46 120,536 5686 0.7
Uranus 19.18 84.07 51,118 866 1.3
Neptune 30.06 164.82 49,660 1030 1.6

Example 5.1

Comparing Densities 

Let’s compare the densities of several members of the solar system. The density of an object equals its mass divided by its volume. The volume ([latex]V[/latex]) of a sphere (like a planet) is calculated using the equation

[latex]\begin{align*}V=\frac{4}{3}\pi{R}^{3}\end{align*}[/latex]

where [latex]\pi[/latex] (the Greek letter pi) has a value of approximately [latex]3.14[/latex]. Although planets are not perfect spheres, this equation works well enough. The masses and diametres of the planets are given in Table 5.2. Let’s use Saturn’s moon Mimas as our example, with a mass of [latex]4\times 10^19[/latex] kg and a diameter of approximately [latex]400[/latex] km (radius, [latex]200\;\text{km}= 2\times 10^5\;\text{m}[/latex]).

Solution

The volume of Mimas is

[latex]\begin{align*}\frac{4}{3}\phantom{\rule{0.2em}{0ex}}\times\phantom{\rule{0.2em}{0ex}}3.14\phantom{\rule{0.2em}{0ex}}\times\phantom{\rule{0.2em}{0ex}}{\left(2\phantom{\rule{0.2em}{0ex}}\times\phantom{\rule{0.2em}{0ex}}{10}^{5}\phantom{\rule{0.2em}{0ex}}\text{m}\right)}^{3}=3.3\phantom{\rule{0.2em}{0ex}}\times\phantom{\rule{0.2em}{0ex}}{10}^{16}\phantom{\rule{0.2em}{0ex}}{\text{m}}^{3}\end{align*}[/latex]

Density is mass divided by volume:

[latex]\begin{align*}\frac{{4\phantom{\rule{0.2em}{0ex}}\times\phantom{\rule{0.2em}{0ex}}10}^{19}\phantom{\rule{0.2em}{0ex}}\text{kg}}{{3.3\phantom{\rule{0.2em}{0ex}}\times\phantom{\rule{0.2em}{0ex}}10}^{16}\phantom{\rule{0.2em}{0ex}}{\text{m}}^{3}}={1.2\phantom{\rule{0.2em}{0ex}}\times\phantom{\rule{0.2em}{0ex}}10}^{3}\phantom{\rule{0.2em}{0ex}}{\text{kg/m}}^{3}\end{align*}[/latex]

Note that the density of water in these units is 1000 kg/m3, so Mimas must be made mainly of ice, not rock.

Exercise 5.1

Calculate the average density of our own planet, Earth. Show your work. How does it compare to the density of an ice moon like Mimas? See Table 5.2 for data.

Solution

For a sphere,

[latex]\begin{align*}\text{density}=\frac{\text{mass}}{\left(\frac{4}{3}\pi\:R^{3}\right)}{\text{kg/m}}^{3}\end{align*}[/latex]

For Earth, then,

[latex]\begin{align*}\text{density}=\frac{6\phantom{\rule{0.2em}{0ex}}\times\phantom{\rule{0.2em}{0ex}}{10}^{24}\phantom{\rule{0.2em}{0ex}}\text{kg}}{4.2\phantom{\rule{0.2em}{0ex}}\times\phantom{\rule{0.2em}{0ex}}2.6\phantom{\rule{0.2em}{0ex}}\times\phantom{\rule{0.2em}{0ex}}{10}^{20}\phantom{\rule{0.2em}{0ex}}{\text{m}}^{3}}=5.5\phantom{\rule{0.2em}{0ex}}\times\phantom{\rule{0.2em}{0ex}}{10}^{3}\phantom{\rule{0.2em}{0ex}}{\text{kg/m}}^{3}\end{align*}[/latex]

This density is four to five times greater than Mimas’. In fact, Earth is the densest of the planets.

Attribution

“7.1 Overview of Our Planetary System” from Douglas College Astronomy 1105 by Douglas College Department of Physics and Astronomy, is licensed under a Creative Commons Attribution 4.0 International License, except where otherwise noted. Adapted from Astronomy 2e.

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Fanshawe College Astronomy Copyright © 2023 by Dr. Iftekhar Haque is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International License, except where otherwise noted.